Transcript for Michael Bloomberg joins Democrats in first debate appearance in Nevada
The new face to face off on the debate stage. Voters getting a first glimpse of how Michael Bloomberg holds up on the debate stage. Already spending millions and rising in the polls, but hoping to find a foothold in the race for the white house. So how did he do? We go to Rick Klein. Any swagger that Mike Bloomberg might have had coming into the debate faded on his wealth, on treatment of women. Yes, he defended himself, fwhu but this is by far the spiciest debate. Senator Sanders looks like he has the most to go forward. Byron? Thank you, Rick. We turn to David Wright, on the highlights, with the front runner facing heat from some of his supporters, accused of bad behavior. Reporter: Tonight in las Vegas, a new contender right in the line of fire. Democrats take a huge risk if we just substitute one arrogant billionaire for another. I don't think you look at Donald Trump and say we need someone richer in the white house. Reporter: For the first time, billionaire businessman Mike Bloomberg was on stage at this NBC debate, dodging the incoming. The mayor says he has a great record. He has not managed his city I have, very well when he was there. Reporter: Bloomberg has been rising in the polls, having spent more than $400 million of his own money mostly on advertising. That makes him a formidable player. They've been sit being out here for ten months, 12 months, raising money, going to caucuses, and all of a sudden he wants to drop in. I know how to take on an arrogant con man who comes from New York. Billionaires today, if you can believe it, have an effective tax rate lower than the middle class. So maybe just -- Tax code. Why are you complaining? Who wrote the code? You and your -- You did, you and your campaign. You and your 99 senators. Not me! He has to prove himself. Welcome to the NBA. He'll get that treatment. This is ridiculous. We're not going to throw out capitalism. Other countries tried that. It was called communism and it just didn't work. Reporter: Elizabeth Warren going on offense. Mayor buttigieg really has a slogan that was thought up by his consultants to paper over a thin version of a plan that would leave millions of people unable to afford their health it's not a plan. It's a power point. And Amy's plan is even less. It's like a post-it note. More Microsoft word guy. I take personal offense since post-it notes were invented in my state. Reporter: Amy klobuchar and buttigieg went out it. You have to own those votes, especially when it comes to immigration. I wish everyone was as perfect as you, Pete. Reporter: Democrats have yet to settle on a moderate candidate in a crowded field. I think Michael Bloomberg is the main beneficiary of Joe Biden's fall and benefitted from the fact that buttigieg and klobuchar have stagnated and settled somehow on Bloomberg but not solidly. Reporter: The billionaire's debut comes as the anti-billionaire Bernie Sanders seems to be locking down the race. Both 78 years old, both taking heat tonight. We shouldn't have to choose between one candidate who wants to burn this party down and another candidate who wants to buy this party out. Reporter: The Vermont senator came into tonight's debate as the undeniable front runner, having won the most votes in new Hampshire. Sanders leads the field by double digits. Sanders built a movement during the 2016 campaign, railing against the billionaire class. Our goal must be a financial system and an economy that works for all of our people. Reporter: But he consistently draws fire for being too fringe. Some of his supporters do, too. Just this weekend, a Bernie supporter got into a fight at a Colorado rally. The altercation captured on camera by CBS's Denver affiliate. There's a whole series of Bernie supporters, called Bernie bros, who seem to cross the line. The Bernie Sanders campaign hates the term Bernie bro. They see it as a vestige of Reporter: Meghan McCain pressed Alexandria ocasio-cortez. The one thing that connect women on left and the right is the abuse subjected to by Bernie it is the most sexist, the most harmful. To a certain extent we have to always reject hate, reject Vitt roll. We also know the amount of anonymous activity that happens on the internet, and that simply is difficult to control. To see calling names, threats. You know, like physically we cannot work anymore. Reporter: Kline has been on the receiving end of some of those attacks. She's the secretary-treasure of the culinary union which did not endorse a candidate this year. That decision plus blunt criticism of medicare for all outraged some Sanders supporters who let loose in a very personal way. They have so many tweets, my daughter sent me three last night, and I said to her, don't worry about it. We're going to be fine. Reporter: Tonight senator Sanders responded. If there are a few people who make ugly remarks who attack trade union leaders I disown those people. They are not part of our Reporter: Tonight the debate at times was raw, especially with Bloomberg there on the stage. You weren't a fan of Obamacare? I am a fan of Obamacare. At the beginning -- Mr. Mayor. I was there. Let me finish, thank you. Reporter: Bloomberg not only has deep pockets, he also has crossover appeal, having served as the Republican mayor of new He says that he has the resources able to take on president trump, but also the temperament to do so. He's witty on social media. His team is creative. And so, with his executive experience in New York and all of that money he's been able to attract a lot of people to his campaign. I'd like to talk about who we're running against, a billionaire who calls women fat broads and horse-faced lesbians. And no, I'm not talking about Donald Trump. I'm talking about mayor Bloomberg. Reporter: He's faced accusations of sexism for what some have described as a corporate culture hostile to women. He's refused to release female executives from non-disclosure agreements in lawsuits they've filed against his company. Are you willing to release all those women from those non-disclosure agreements so we can hear their side of the story? We have a very few non-disclosure agreements. How many is that? Let me finish. How many is that? None of them accuse me of doing anything other than maybe they didn't like a joke I told. And let me just, and let me, there's agreements between two parties that wanted to keep it quiet, and that's up to them. Reporter: Bloomberg's also haunted by his long-time support of the NYPD stop and frisk policy which targeted minorities. But as he prepared to run for president he apologized for stop and frisk. I was wrong. Reporter: Tonight the issue was still front and center. Stop and frisk which went after after can American and Latino people in an outrageous way. The one thing that I'm really worried about, embarrassed about, was how it turned out with stop and frisk. The democratic voters I talked to say they use these debates as basically a gut check. We'll see in the coming weeks if voters say they could picture mayor Bloomberg standing next to president trump in the fall. Reporter: Tonight president trump offered hints as to who he sees as the biggest threats at a campaign rally 300 miles agray Las Vegas. He took aim at Bloomberg. I hear he's getting pounded tonight. You know he's in a debate. Reporter: And Sanders. The DNC is going to take it away from Bernie again. And that's okay. We don't care who the hell it is, we're going to win. Reporter: Tonight's debate and Saturday's caucuses may help to decide who does face him. I'm David Wright for "Nightline."
This transcript has been automatically generated and may not be 100% accurate.